For Some Distilleries, Worm Tubs Leave Just the Right Amount of Sulfur

Most Scottish distilleries use pot stills, which are relatively simple: beer goes in, heat is applied, spirit comes out. But there are a few different strategies distillers can use for that final step of re-condensing the alcohol vapor into liquid again.

The most common piece of equipment is called a shell-and-tube condenser. It looks something like its name suggests: a copper shell, criss-crossed internally by hundreds of copper tubes. When the still is operational, the tubes are filled with cool water. The vapor is directed through the shell, where it encounters those chilled copper tubes and re-condenses, falling back into a liquid state.

An older strategy, but one still used at a handful of distilleries today, is the worm tub. A worm tub consists of a tank of cold water with a spiral copper pipe – something like a gigantic Slinky – running through it. (If you’re a homebrewer, this looks a lot like the plumbed copper spiral you might use to cool your wash). Alcohol vapor travels through the pipe, condensing on the cold copper surface all the way, and by the end, only liquid alcohol emerges.

Mortlach distillery is famous for its “meaty” flavor profile – due in part to using worm tubs instead of shell-and-tube condensers. (image via Andrew Wood, CC BY-SA 2.0)

Both systems work to cool vapor back to a liquid state, but shell-and-tube condensers provide much more contact with copper than a worm tub does – Bull Lumsden, director of distilling at Glenmorangie, estimates as much as 20 times more. One of copper’s main roles in distillation is to pull sulfur out of the distillate, so more copper contact equals a lighter, fresher flavor profile.

But light and fresh isn’t necessarily what every distillery is looking for. Many Scotch distilleries prefer the heavier, more savory flavor in their new make that comes from more sulfur molecules making their way out of the distillation system. Worm tub condensers are one way to get that weightiness. Some of the distilleries still using worm tubs include Mortlach, Knockdhu, Benrinnes, Glen Elgin, Oban, Speyburn, Glenkinchie, Dalwhinnie, and Royal Lochnagar, among others.